Microns for



March 31, 1964 c. G. CHADWICK 3,127,283

PROCESS FOR PRODUCING A METAL CASE Filed Oct. 2, 1959 (I) MAINTAIN AT IOOO C. TO I500C. AND 2000 MICRONS FOR 20 TO 60 HOURS (2) COOL TO SOO'C OR LOWER AND INCREASE PRESSURE TO ATMOSPHERIC STEEL PLATE REFRACTORY SUPPORT CHROME CASNG AGENT LINED FLAT CAR- VACUU M FURNACE INVENTOR. CECIL G. CHADWICK A TTORNEY United rates Patent 3 127 283 rnocuss non rnoourznvt; A METAL CASE Cecil George (Ihadwicir, Niagara Fmls, N.Y., assignor to Union Carbide Corporation, a corporation of New York Filed Get. 2, 1959, Ser. No. 843,934 9 Claims. (Cl. 117-406) This invention relates to a process for providing a base material with a metal or silicon casing by diffusion of the metal or silicon casing into the base material.

While each of the known metals possess certain desirable properties, in general, the desirable properties are often accompanied by certain undesirable properties. For example, some alloy steels possess good high-temperature stress-rupture properties but are lacking in high-temperature oxidation resistance. Metals, such as chromium, generally possess good high-temperature oxidation resistance but lack in high-temperature stress-rupture properties. A combination of these properties would be desirable.

Attempts have been made to obtain such a desirable combination of properties by coating a metal or alloy with other metals. Various processes, such as electrodeposition, dipping, and cementation have been employed to apply a coating to a base material. These processes, however, have not been altogether successful.

Although electrodeposition of a metal to a base material has been successful for certain purposes, electrodeposited metal is not free from pores and, thus, does not provide a continuous coating. In addition, electrodeposited metal generally has a sharply defined interface with the underlying base material and severe stresses resulting from differential expansion at the interface frequently lead to spalling of the coating, especially during heating and cooling.

Cementation processes, by which a metal coating is deposited on a base material by heating at a high temperature while the surface of the metal to be coated is in intimate contact with the coating metal, is well known (US. Patent No. 1,365,499, to Kelley). This process has met with varying degrees of success; however, certain inherent disadvantages exist in this process. For example, the intimate contact between the base material and the coating metal causes a sintering of the materials. The sintering of the coating metal to the substrate results in a non-uniform and porous metallic coating. Upon exposure of the coated material to various environments, the base material will be damaged despite the coating applied thereto. In addition, the porous coating also presents a sharply defined interface between the coating maerial and base material where cracking and spalling can readily occur.

In an attempt to avoid the use of cementation processes, it was found that a coating could be applied by bringing into contact with the surface of the base material a volatile halide of the coating metal under suitable conditions of temperature and pressure whereby a metal coating is applied to the base material.

Although, this process met with a certain degree of success, certain inherent disadvantages exist. For example, upon completion of the coating step, the resultant coated material must be washed in hot water and then boiled in an appropriate solution to neutralize and remove any salts adhering thereto.

It would be desirable to be able to provide base metals with alloyed casings of other metals and silicon whereby the resulting product would have the surface properties of the casing alloy while retaining the over-all properties of the base metal. Such a casing would require relatively deep penetration of the casing material into the base metal if a durable and effective casing is to be obtained.

ice

The casing should be substantially uniform and the cased product should exhibit no serious change in dimensions from those of the base material.

It is an object of this invention, therefore, to provide a process which is capable of providing base metals with a suitable alloyed casing.

Other objects will be apparent from the subsequent disclosure and appended claims and the drawing which shows somewhat schematically the physical arrangement of material and equipment for a particular embodiment of the present invention.

The process which satisfies the objects of the present invention comprises introducing into a casing zone the metal substrate to be treated; introducing into a vaporizing zone a source of the casing material, said casing material being maintained out of contact with the substrate; heating the source of casing material at a reduced pressure to an elevated temperature to produce vapor of the casing material, the material atoms of which vapor have sufficient energy to traverse the distance between the substrate and the source of the casing material, the temperature of the vapor being at least that at which the substrate and the casing material exhibit substantial mutual affinity; heating the substrate in the casing zone under a reduced pressure not exceeding the pressure in the vaporizing zone at least to a temperature at which the substrate and the casing material exhibit substantial mutual affinity but below the temperature at which the substrate melts and below the temperature at which the vapor pressure of absorbed casing material at the surface of the substrate is equal to the vapor pressure of the casing material at the temperature of the vapor of said casing material; intimately contacting the substrate with the casing material vapor; and maintaining the substrate in intimate contact with the casing material vapor until the vapor has advanced into the substrate to a predetermined depth.

As employed herein the term mutual affinity includes either the ability of the materials to form solid solutions or to form intermetallic compounds.

The vaporizing zone and casing zone for the aforementioned process may be co-extensive. In such a situation, the pressure and temperature in the two zones will, of course, be the same but still must meet the previously mentioned limitations.

For the purposes of clarity, the process of the present invention will be discussed in terms of the applying of a chrome casing (hereinafter referred to as chrome casing) to steels. The process will also be described in terms of the co-extensive zoning, i.e., the vaporizing zone and the casing zone being one and the same. With reference to the drawing, the articles to be chrome cased may be placed on a suitable conveying mechanism such as, for example, a refractory-lined flat car. The chrome source, i.e., the casing metal, is placed in close proximity to but out of contact with the steel articles. The chrome casing agent and the steel articles may be stacked out of contact with each other in rows on this flat car. It is also possible to place a layer of the chrome casing agent on the flat car and employ refractory supports to hold the articles to be chrome cased out of contact with the chrome casing agent as shown in the drawing.

The loaded flat car may then be placed in a large, horizontal-type vacuum furnace and heated to a suitable temperature, for example, about 1000 to 1500 C. at a suitable pressure, for example, up to about 2000 microns of mercury. The heating is continued until. the desired depth of penetration is obtained. At the exemplified temperatures and pressures, heating for about twenty to sixty hours has been found to provide a good chrome casing for steel. The furnace may then be cooled to about 300 C. or lower, the pressure adjusted to about atmospheric and the flat car and its contents removed.

When such a process has been employed, employing a ferrochromium alloy as the source of chromium casings, bright in color and having thicknesses of from about 0.031 inch to about 0.125 inch thick, have been obtained. The composition of the casing at the surface of the chrome cased plates was high in chrome with decreasing chrome content toward the center of the chrome cased late.

p The source of chrome vapor may be suitable chrome metal or ferrochromium alloys. Other forms which have been found suitable include mixtures of high-carbon ferrochromium alloys with various oxidants such as chrome ore, nickel oxide, silica, iron oxide, and oxidized ferrochromium. Such mixtures may be suitably employed in the form of pellets, briquette or other lump form. Suitable binding agents such as chromic acid, molasses, Mogul, or glucose with water may be employed.

In an example of the invention approximately 1000 parts by weight of comminuted high-carbon ferrochromium alloy having the approximate composition of 66.3 weight percent chromium, 24.8 Weight percent iron, 4.75 weight percent carbon, 0.055 weight percent sulphur and 1.7 Weight percent silicon were mixed with 337 parts by weight of Tiebaghi chrome ore containing 54.23 weight percent Cr O 14.69 percent FeO, 3.64 weight percent SiO 10.62 weight percent A1 and 15.68 weight percent combined magnesium and calcium oxides and compressed with 21 parts by Weight of Mogul and 85 parts by weight of water into oval shaped pellets approximately 1.25 inch long by 0.875 inch wide by 0.725 inch thick. The pellets were dried and charged on to a railroad flat car with the articles to be chrome cased but out of contact with them. The articles were %-inch by 15- inch by 30-inch steel plates. The loaded flat car was placed in a vacuum furnace and heated under a reduced pressure of about 100 to 2000 microns of mercury absolute for approximately 50 to 60 hours at a temperature of between 1000 and 1385 C. Casings of chromium 0.08 inch in depth and averaging 20 percent chromium were obtained. (Surface averaged about 35 percent.)

As was stated previously, the process is not limited to the chrome casing of steel. Silicon may also be equally well employed. In an example of the application of the silicon casing to steel, a small quantity of crushed silicon (through 30, on 80 mesh) was placed in a ceramic (A1 0 boat. A strip of sandblasted 1020 steel was placed across the boat completely out of contact with the silicon metal. The boat and steel were then placed in a furnace which was heated to 1200 C. for 12 hours and then heated to 1350 C. for an additional 12 hours. In the course of the heating, the furnace pressure was maintained at about 25 microns. Upon conclusion of the heat treatment, the surface of the steel was spectrographically analyzed and was found to contain from about 1 percent to 10 percent silicon. By this means, silicon casings to a depth of 0.1 inch had been obtained. Following this same procedure, nickel has been introduced satisfactorily into steel to provide steel surfaces containing from 1 percent to 10 percent nickel.

The substrates suitable for treatment according to the process of the present invention are not limited to steel. To illustrate this, vanadium, tantalum, columbium, and molybdenum have been treated with metal vapor from vacuum-grade chromium metal pellets in a furnace which Was then heated for 2 hours at 1200 C. and for an additional 12 hours at 1300 C. while maintaining the furnace pressure at about microns. About 0.1 gram of chromium was produced as a casing for the 13.7 gram vanadium sample. A casing of 0.31 gram of chromium was produced for the 19.24 gram sample of tantalum. Similar casings were produced on the columbium and molybdenum specimens. Chromium has been introduced into Hastelloy turbine plates and Haynes 21 alloy (Hastelloy and Haynes are trademarks of the Union Carbide Corporation.) The furnace was held at 1150 C. and SO-microns pressure for 24 hours. Vacuumgrade chromium pellets when heated for 24 hours at a temperature of 1350 C. at a pressure of 50 microns increased the chromium content of stainless steel from 16.68 percent chromium to 17.97 percent.

Cast iron specimens were heated for 18 hours at 1100 C. in the presence of vacuum-grade chrome pellets at a pressure of 50 microns. The chromium penetration in the specimens averaged 0.035 inch.

There is no limitation on the shape or form of the substrate which can be treated according to the process of the present invention. Iron powder of a particle size of through mesh has been provided with a chromium content of 17.0 percent.

In accordance with the process of the present invention, chromium may be introduced very satisfactorily into steel, iron, and nickel. In addition, chromium may be readily introduced into molybdenum and tungsten as Well as in the metals shown in the previous examples. Boron may be readily introduced into steel and manganese and silicon may be introduced into steel, iron, and nickel. Iron may be introduced as a casing for other metals but is of limited application. Titanium may be introduced into substrates but requires high-vacuum nonoxidizing conditions. Nickel has a very small vapor pressure at its melting point. Hence, it would normally be necessary to use molten nickel to obtain sufficient vapor for high transfer rates. In addition, beryllium and palladium may be satisfactorily transferred as a casing to other metals.

If the source of casing metal and the specimen are permitted to come into intimate contact, the sintering of the two results in a scab formation on the specimen. The resulting product must be drastically machined to reach a uniform surface. The products produced according to the process of the present invention require merely a polishing to highlight the case alloy surfaces.

Additionally, when intimate contact is maintained the casing which results tends to be nonuniform whereas the process of the present invention provides a substantially uniform casing. As an example of the utility of the present invention, plates of chrome cased steel have been rolled and sheared into bumper blanks, 7 inches wide and 66 inches long, and polished. The bumpers were quite satisfactory and, thus, it may be seen that chrome casing may be employed to produce bumpers with a strong, highly polished surface.

What is claimed is:

1. A process for providing a metal containing substrate with a diffused and continuous casing of a material selected from the group consisting of chromium, silicon, boron, manganese, nickel, beryllium, titanium and palladium which comprises introducing into a casing zone the metal substrate to be treated; introducing into a vaporizing zone a source of easing material; said source of casing material being spaced away from said substrate; heating said source of casing material at a reduced pressure to an elevated temperature to produce vapor of said casing material, the atoms of which vapor have sufiicient energy to traverse the distance between said substrate and said source of said casing material, the temperature of said vapor being at least that at which said substrate and said casing material exhibit substantial mutual aflinity; heating said substrate in said casing zone under a reduced pressure not exceeding the pressure in said vaporizing zone at least to a temperature at which said substrate and said casing material exhibits substan-' tial mutual aflinity but below the temperature at which said substrate melts and below the temperature at which the vapor pressure of absorbed casing material at the surface of said substrate is equal to the vapor pressure of the casing material at the temperature of said vapor of said casing material; intimately contacting said substrate with said casing material vapor; and maintaining said substrate in intimate contact with said casing material vapor until the material vapor has diffused into said substrate to a predetermined depth.

2. A process in accordance with claim 1 wherein said casing zone and said vaporizing zone are co-extensive.

3. A process in accordance with claim 1 wherein said casing material is silicon.

4. A process in accordance with claim 1 wherein said casing material is nickel.

5. A process in accordance with claim 1 wherein said casing material is manganese.

6. A process in accordance with claim 1 wherein said casing material is titanium.

7. A process in accordance with claim 1 wherein said casing material is palladium.

8. A process for providing steel with an alloyed chromium casing which comprises introducing into a vacuum furnace chromium metal and the steel to be cased, said chromium and said steel being out of contact with each other; heating said furnace to a temperature in the range of about 1000 to 1500 C. at a pressure not exceeding 2000 microns of mercury; and maintaining said furnace at said temperature and pressure until chromium vapor is produced and diffuses into said steel to a predetermined depth.

9. A process for providing a steel article with an alloyed chromium casing which comprises arranging in References Cited in the file of this patent UNITED STATES PATENTS 1,224,339 Darrah et al May 1, 1917 1,899,569 Howe Feb. 28, 1933 2,413,605 Colbert et a1 Dec. 31, 1946 2,479,541 Osterberg Aug. 16, 1949 2,610,606 Weber et al Sept. 16, 1952 2,809,127 Gibson Oct. 8, 1957 2,822,301 Alexander et a1. Feb. 4, 1958 OTHER REFERENCES Diffuse Coatings on Iron and Stee Gorbunor, The Academy of Sciences of the USSR, Moscow, 1958, pp. 21-29 relied on.

Vacuum Technique (Dushman), published by John Wiley and Sons, Inc. (New York), 1949 (pages 762 and 763 relied on). 

1. A PROCESS FOR PROVIDING A METAL CONTAINING SUBSTRATE WITH A DIFFUSED AND CONTINUOUS CASING OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OF CHROMIUM, SILICON, BORON, MANGANESE, NICKEL, BERYLLIUM, TITANIUM AND PULLADIUM WHICH COMPRISES INTRODUCING INTO A CASING ZONE THE METAL SUBSTRATE TO BE TREATED, INTRODUCING INTO A VAPORIZING ZONE A SOURCE OF CASING MATERIAL SAID SOURCE OF CASING MATERIAL BEING SPACED AWAY FROM SAID SUBSTRATE, HEATING SAID SOURCE OF CASING MATERIAL OF A REDUCED PRESSURE TO AN ELEVATED TEMPERATURE TO PRODUCE VAPOR OF SAID CASING MATERIAL, THE ATOMS OF WHICH VAPOR HAVE SUFFICIENT ENERGY TO TRAVERSE THE DISTANT BETWEEN SAID SUBSTRATE AND SAID SOURCE OF SAID CASING MATERIAL, THE TEMPERATURE OF SAID VAPOR BEING AT LEAST THAT AT WHICH SAID SUBSTRATE AND SAID CASING MATERIAL EXHIBIT SUBSTANTIAL MUTUAL AFFINITY; HEARING SAID SUBSTRATE IN SAID CASING ZONE UNDER A REDUCED PRESSURE NOT EXCEEDING THE PRESSURE IN SAID VAPORIZING ZONE AT LEAST TO A TEMPERATURE OF WHICH SAID SUBSTRATE AND SAID CASING MATERIAL EXHIBITS SUBSTANTIAL MUTUAL AFFINITY BUT BELOW THE TEMPERATURE OF WHICH SAID SUBSTRATE MELTS AND BELOW THE TEMPERATURE AT WHICH THE VAPOR PRESSURE OF ABSORBED CASING MATERIAL AT THE SURFACE OF SAID SUBSTRATE IS EQUAL TO THE VAPOR PRESSURE OF THE CASING MATERIAL AT THE TEMPERATURE OF SAID VAPOR OF SAID CASING MATERIAL; INTIMATELY CONTACTING SAID SUBSTRATE WITH SAID CASING MATERIAL VAPOR, AND MAINTAINING SAID SUBSTRATE IN INTIMATE CONTACT WITH SAID CASING MATERIAL VAPOR UNTIL THE MATERIAL VAPOR HAS DIFFUSED INTO SAID SUBSTRATE TO A PREDETERMINED DEPTH. 